The present invention relates to a steplessly variable transmission for automotive vehicles, and more particularly to a speed ratio control of an automotive vehicle transmission having a belt-pully type, steplessly variable transmission mechanism.
One known type of steplessly variable transmission has belt-pulley transmission mechanism which includes a pair of, or primary and secondary, pulleys variable in pitch or effective diameter and an endless belt rotationally coupling the pair of pulleys together. The primary pulley is connected to an engine output and the secondary pulley to a drive line. Either one or both of the primary and secondary pulleys are changeable in effective diameter to steplessly vary the speed ratio of the belt-pulley transmission mechanism. Such a steplessly variable transmission is disclosed in, for example, Japanese Patent Publication No. 62-52,176 entitled "Speed Ratio Control For A Steplessly Variable Vehicle Transmission", published Nov. 4, 1987. In the steplessly variable transmission disclosed by the above noted Japanese Patent Publication, the speed ratio of the belt-pulley transmission mechanism is controlled in such a way as to change the speed ratio of the belt-pulley transmission mechanism so as to vary an engine speed of the vehicle engine following predetermined or preselected speed ratio change schedules of a target engine speed relative to a vehicle speed at various throttle openings of an engine throttle between full throttle and idle throttle.
Steplessly variable transmissions are often equipped with a drive mode change feature allowing the drive mode to be changeable in operation between an economy drive mode, in which the vehicle acceleration is relatively moderate for fuel economy, and a power drive mode, in which the vehicle is provided with power and is accelerated. In a steplessly variable transmission of this type, maps of speed ratio change schedules, one of which is provided for each of various engine loads or throttle openings, have different patterns in the economy and power drive modes. For example, the above noted Jananese Patent Publication discloses the maps of speed ratio change schedules as having different patterns in the power and economy drive modes. The speed ratio change schedules are different at an upper engine load limit, namely at full throttle, but are identical at a lower engine load limit or an idle throttle. In more detail, the speed ratios at various throttle openings, including a full throttle opening, are generally established so that they are higher over the whole range of vehicle speeds for the power drive mode than they are for the economy drive mode. According to this prior steplessly variable transmission, the vehicle engine fuel consumption is improved, since an increase in engine speed is suppressed in the economy drive mode. On the other hand, engine output power is also improved, since engine speed can be increased higher in the power drive mode.
The biggest drawback to the use of such maps of speed ratio change schedules is that, because the speed of a vehicle engine is suppressed so as to be relatively low at a full throttle in the economy drive mode, it is hard to accelerate the vehicle sufficiently at a full throttle. This leads to a lack of engine output over a range of relatively large throttle openings. Also, because the speed of the vehicle engine can still become higher, the fuel performance takes a turn for the worse if a high engine output is not necessary.